U.S. patent application number 12/170715 was filed with the patent office on 2009-06-04 for apparatus and method for compensating carrier feedthrough in quadrature modulation system.
This patent application is currently assigned to Electronics and Telecommunications Research Institute. Invention is credited to Byung Su KANG, Joon Hyung KIM, Heon Kook KWON, Kwang Chun LEE.
Application Number | 20090140821 12/170715 |
Document ID | / |
Family ID | 40675103 |
Filed Date | 2009-06-04 |
United States Patent
Application |
20090140821 |
Kind Code |
A1 |
KWON; Heon Kook ; et
al. |
June 4, 2009 |
APPARATUS AND METHOD FOR COMPENSATING CARRIER FEEDTHROUGH IN
QUADRATURE MODULATION SYSTEM
Abstract
The present invention relates to an apparatus and a method for
compensating carrier feedthrough in a quadrature modulation system.
In order to suppress the carrier feedthrough, and minimize and
compensate the carrier feedthrough, differences of baseband
differential input DC voltages in an in-phase as well as a
quadrature-phase are simultaneously adjusted to 0 or a certain
slight voltage difference by a simple analog circuit. Therefore, it
is possible to suppress carrier feedthrough using a simple analog
type apparatus for compensating carrier feedthrough, and simply
achieve an apparatus for carrier feedthrough using a variety of
quadrature modulators.
Inventors: |
KWON; Heon Kook; (Daejeon,
KR) ; KIM; Joon Hyung; (Daejeon, KR) ; KANG;
Byung Su; (Daejeon, KR) ; LEE; Kwang Chun;
(Daejeon, KR) |
Correspondence
Address: |
Jefferson IP Law, LLP
1730 M Street, NW, Suite 807
Washington
DC
20036
US
|
Assignee: |
Electronics and Telecommunications
Research Institute
Daejeon
KR
Samsung Electronics Co., LTD.
Suwon-city
KR
|
Family ID: |
40675103 |
Appl. No.: |
12/170715 |
Filed: |
July 10, 2008 |
Current U.S.
Class: |
332/103 |
Current CPC
Class: |
H03C 3/406 20130101;
H04L 27/364 20130101 |
Class at
Publication: |
332/103 |
International
Class: |
H03C 1/52 20060101
H03C001/52; H03C 1/00 20060101 H03C001/00; H04L 27/36 20060101
H04L027/36 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 4, 2007 |
KR |
10-2007-0125060 |
Claims
1. An apparatus of compensating carrier feedthrough in a quadrature
modulation system, comprising: a quadrature modulator that outputs
carrier feedthrough signals generated by a difference between a
first bias voltage and a first control voltage in an in-phase path
and a difference between a second bias voltage and a second control
voltage in a quadrature-phase; a coupler that extracts a portion of
the carrier feedthrough signals from the quadrature modulator; a
detector that measures an amount of carrier feedthrough by
detecting a magnitude of the extracted carrier feedthrough signals
and converts the carrier feedthrough signals into a DC signal; and
controllers that output the first control voltage and the second
control voltage by comparing the output DC signal with a first
reference voltage and a second reference voltage, respectively.
2. The apparatus of claim 1, wherein the controller comprises: a
first filter that removes the periodicity of the DC signal and
outputs the DC signal; a second filter that removes the periodicity
of the DC signal and outputs the DC signal; a first comparator that
outputs a first compared value by comparing the DC signal with the
periodicity removed with the first reference voltage; a second
comparator that outputs a second compared value by comparing the DC
signal with the periodicity removed with the second reference
voltage; a first integrator that receives and accumulates the first
compared values output from the first comparator, and generates the
first control voltage; a second integrator that receives and
accumulates the second compared values output from the second
comparator, and generates the second control voltage; a first
switching unit that receives the DC signal with the periodicity
removed by the first filter and a compensation control signal,
which is a control signal for compensating carrier feedthrough with
respect to an in-phase signal and a quadrature-phase signal applied
through the in-phase path and the quadrature-phase path,
respectively, and applies a first reference voltage to the first
comparator on the basis of the DC signal with the periodicity
removed by the first filter and the compensation signal; and a
second switching unit that receives the DC signal with the
periodicity removed by the second filter and a compensation control
signal, and applies a second reference voltage to the second
comparator on the basis of the signal with the periodicity removed
by the second filter and the compensation control signal.
3. The apparatus of claim 1, further comprising a switching unit
that alternately provides the DC signal output from the detector to
the controllers, in accordance with a predetermined period
4. The apparatus of claim 3, wherein the switching unit comprises a
first switching unit that switches connection of the DC signal from
the detector to the controllers; and a second switching unit that
outputs the DC signal to the controller in the predetermined
period, and outputs information about the amount of carrier
feedthrough to the controllers.
5. The apparatus of claim 1, wherein the first control voltage is a
voltage needed to adjust a voltage difference between a first DC
signal representing an amount of carrier feedthrough generated in
the in-phase path and the first reference voltage, and the second
control voltage is a voltage needed to adjust a voltage difference
between a second DC signa representing an amount of carrier
feedthrough in the quadrature-phase path and the second reference
voltage.
6. The apparatus of claim 5, wherein the first reference voltage is
a control voltage that minimizes the amount of carrier feedthrough
in the in-phase path, and the second reference voltage is a control
voltage that minimizes the amount of carrier feedthrough in the
quadrature-phase.
7. The apparatus of claim 1, further comprising: a first switching
unit that switches a path to receive one of a first input signal
and a certain signal in the in-phase path, depending on the state
of a compensation control signal; a second switching unit that
switches a path to receive one of a second input signal and a
certain signal in the quadrature-phase path, depending on the state
of the compensation control signal; a differential amplifier that
transmits the first control voltage and the second control voltage
output from the controllers and transmits the first bias voltage
and the second bias voltage to the quadrature modulator; and a
power amplifier that receives a signal output from the quadrature
modulator, amplifies the power of the signal, and outputs the
signal.
8. A method of compensating carrier feedthrough in a quadrature
modulation system, comprising: detecting a carrier feedthrough
signal with respect to an in-phase path and a quadrature-phase path
on the basis of a first bias voltage and a second bias voltage;
determining whether a DC voltage with respect to the detected
carrier feedthrough signal agrees with a first reference voltage
and a second reference voltage; switching the path to apply the
same reference voltages to comparators while blocking a feedback
signal, when the DC voltage agrees with the first reference voltage
and the second reference voltage; and generating the first control
voltage and the second control voltage using the same reference
voltages, and applying the generated first control voltage and
second control voltage to a signal in the in-phase path and a
signal in the quadrature-phase path, respectively.
9. The method of claim 8, wherein the detecting comprises:
calculating an amount of carrier feedthrough with respect to the
carrier feedthrough signal and converting the carrier feedthrough
signal into a DC voltage; alternately outputting the DC voltage in
a predetermined period; outputting one of the DC voltages that are
alternately output after removing the periodicity of the DC
voltage; and comparing the DC voltage with the periodicity removed
with the first reference voltage and the second reference voltage,
and generating the first control voltage and the second control
voltage from the compared results.
10. The method of claim 8, further comprising: generating a first
control voltage by integrating differences of the first DC voltage
that representing an amount of carrier feedthrough in the in-phase
path and the first reference voltage, when it is determined that
the first DC voltage does not agree with the first reference
voltage in the determining; and generating a second control voltage
by integrating differences of the second DC voltage that
representing an amount of carrier feedthrough in the
quadrature-path and the second reference voltage, when it is
determined that the second DC voltage does not agree with the
second reference voltage.
11. The method of claim 10, wherein the first control voltage is a
voltage needed to adjust a voltage difference between the first DC
voltage and the first reference voltage, and the second control
voltage is a voltage needed to adjust a voltage difference between
the second DC voltage and the second reference voltage.
12. The method of claim 11, wherein differences between the first
control voltage and the first bias voltage, and the second control
voltage and the second bias voltage are set to one of 0 or a
predetermined value, on the basis of the first reference voltage
and the second reference voltage.
13. An apparatus for compensating carrier feedthrough, comprising:
a coupler that detects a portion of a carrier feedthrough signal
with respect to a certain signal on the basis of a bias voltage; a
detector that measures an amount of carrier feedthrough by
detecting the magnitude of a feedthrough carrier with respect to
the carrier feedthrough signal detected by the coupler, and
converting the carrier feedthrough signal into a DC signal; a
controller that compares the DC signal with a predetermined
reference voltage and then outputs a control voltage; and a
differential amplifier that applies the control voltage output from
the controller and the bias voltage.
14. The apparatus of claim 13, further comprising: a switching unit
that switches a path to receive one of an input signal and the
certain signal, depending on the state of a compensation control
signal; a quadrature modulator that receives the control voltage
and a certain signal from the differential amplifier and outputs
the control voltage and the certain signal into a quadrature
modulation signal; and a power amplifier that receives the
quadrature modulation signal output from the quadrature modulator,
amplifies the power of the quadrature modulation signal, and then
outputs the quadrature modulation signal.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to and the benefit of
Korean Patent Application No. 10-2007-0125060 filed in the Korean
Intellectual Property Office on Dec. 4, 2007, the entire contents
of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] (a) Field of the Invention
[0003] The present invention relates to an apparatus and a method
for compensating carrier feedthrough in a quadrature modulator and
a quadrature modulation system.
[0004] (b) Description of the Related Art
[0005] In general, an ideal voltage difference in a quadrature
modulator is considered to be when the DC voltage difference of a
baseband differential input is 0, since this prevents carrier
feedthrough. However, the DC voltage difference of the baseband
differential input in the quadrature modulator is not substantially
0, and this causes carrier feedthrough due to mixing with a local
oscillator signal.
[0006] Currently, carrier feedthrough occurs even in a quadrature
modulator used in a system using quadrature modulation, the carrier
feedthrough being caused by the DC voltage difference of a baseband
differential input. The carrier feedthrough occurs differently as
various types of the quadrature modulators. For example, there are
quadrature modulators of which the carrier feedthrough is at the
minimum when the DC voltage difference is 0, whereas there are
quadrature modulators of which the carrier feedthrough is at the
minimum when a slight DC voltage difference appears.
[0007] A digital compensating circuit has been used in the related
art to suppress the carrier feedthrough. In more detail, it detects
a carrier feedthrough first, digital circuit compensating carrier
feedthrough is activated by comparator and the carrier feedthrough
is compensated by a DC compensation signal through a
digital-to-analog converter. However, according to the above
method, because a number of digital blocks, i.e., constituent
elements including a comparator, a digital-analog converter, a
control signal generator, etc., in addition to the quadrature
modulator are needed, the configuration of the system becomes
complicated.
[0008] Another method has been disclosed in the related art that
detects a DC voltage error in an in-phase/quadrature-phase input in
a modulator, and then compensates carrier feedthrough using a bias
current of a transistor that performs a modulating operation.
However, this method can be applied to only the design of a
quadrature modulator.
[0009] Another method has been disclosed in the related art that
suppresses carrier feedthrough at in-phase/quadrature-phase using a
correlator, an integrator, and a pseudo-noise generator on a
feedback path. However, the configuration is too complicated to
implement a device that suppresses carrier according to this
method.
SUMMARY OF THE INVENTION
[0010] The present invention has been made in an effort to provide
an apparatus for compensating carrier feedthrough that is generated
by a modulator output or a transmitter's output in a quadrature
modulation system.
[0011] Further, the present invention provides a method of
adjusting a DC voltage difference using the apparatus for
compensating carrier feedthrough.
[0012] In order to accomplish the technical objects of the
invention, an apparatus for compensating carrier feedthrough in a
modulation system, includes: a quadrature modulator that outputs
carrier feedthrough signals generated by a difference between a
first bias voltage and a first control voltage in an in-phase path
and a difference between a second bias voltage and a second control
voltage in a quadrature-phase; a coupler that extracts a portion of
the carrier feedthrough signals from the quadrature modulator; a
detector that measures the amount of carrier feedthrough by
detecting the magnitude of the extracted carrier feedthrough
signals and converts the carrier feedthrough signals into a DC
signal; and controllers that output the first control voltage and
the second control voltage, respectively, by comparing the output
DC signal with the first reference voltage and the second reference
voltage.
[0013] In order to accomplish the technical objects of the
invention, a method of compensating carrier feedthrough in a
quadrature modulation system includes: detecting a carrier
feedthrough signal with respect to an in-phase path and a
quadrature-phase path on the basis of a first bias voltage and a
second bias voltage; calculating the amount of carrier feedthrough
signal and converting the carrier feedthrough signal into a DC
voltage; alternately outputting the DC voltage in a predetermined
period; outputting any one of the DC voltages that are alternately
output after removing the periodicity of the DC voltage; comparing
the DC voltage with the periodicity removed with the first
reference voltage and the second reference voltage, and generating
the first control voltage and the second control voltage from the
compared results; determining whether a DC voltage with respect to
the detected carrier feedthrough signal agrees with a first
reference voltage and with a second reference voltage; switching
the path to apply the same reference voltages to a comparator while
blocking a feedback signal, when the DC voltage agrees with the
first reference voltage and the second reference voltage; and
generating the first control voltage and the second control voltage
using the same reference voltage, and applying the generated first
control voltage and second control voltage to a signal in the
in-phase path and a signal in the quadrature-phase path,
respectively.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a diagram illustrating the configuration of an
apparatus for compensating carrier feedthrough according to an
exemplary embodiment of the present invention.
[0015] FIG. 2 is a diagram illustrating the detailed configuration
of an apparatus for compensating carrier feedthrough according to
an exemplary embodiment of the present invention.
[0016] FIG. 3 is a circuit diagram of an apparatus for compensating
carrier feedthrough according to an exemplary embodiment of the
present invention.
[0017] FIG. 4 is a flowchart illustrating a method of compensating
carrier feedthrough according to an exemplary embodiment of the
present invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0018] In the following detailed description, only certain
exemplary embodiments of the present invention have been shown and
described, simply by way of illustration. As those skilled in the
art would realize, the described embodiments may be modified in
various different ways, all without departing from the spirit or
scope of the present invention. Accordingly, the drawings and
description are to be regarded as illustrative in nature and not
restrictive. Like reference numerals designate like elements
throughout the specification.
[0019] It will be further understood that the terms "comprises"
and/or "comprising", when used in this specification, specify the
presence of stated components, but do not preclude the presence or
addition of one or more other components, unless specifically
stated. In addition, the terms "-er" , "-or" and "module" described
in the specification mean units for processing at least one
function and operation and can be implemented by hardware
components or software components, and combinations thereof.
[0020] Throughout this specification and the claims that follow,
when it is described that an element is "coupled" to another
element, the element may be "directly coupled" to the other element
or "electrically coupled" to the other element through a third
element.
[0021] FIG. 1 is a diagram illustrating the configuration of an
apparatus for compensating carrier feedthrough according to an
exemplary embodiment of the present invention.
[0022] As shown in FIG. 1, an apparatus for compensating carrier
feedthrough includes a plurality of switching units 100, 110, 700,
and 800, a first differential amplifier 200, a second differential
amplifier 210, a quadrature modulator 300, a power amplifier 400, a
coupler 500, a detector 600, a first controller 900, and a second
controller 910.
[0023] First, the plurality of switching units 100, 110, 700, and
800 integrally means a first switching unit to a fourth switching
unit. The first switching unit 100 switches a path between
grounding and an in-phase signal baseband signal by control of a
compensation signal (compensation on/off), and the second switching
unit switches a path between grounding and a quadrature-phase
signal of the baseband signal by control of a compensation
signal.
[0024] In detail, when an on-state compensation control signal is
input, the first switching unit 100 and the second switching unit
110 are grounded, and when an off-state compensation control signal
is input, the first switching unit 100 and the second switching
unit 110 are connected such that they can receive an in-phase
baseband signal and a quadrature-phase baseband signal,
respectively.
[0025] Further, the third switching unit 700 switches a carrier
feedthrough signal detected by the detector 600 to the first
controller 900 and the second controller 910 by control of a
compensation control signal. The carrier feedthrough signal is a
signal converted into a DC voltage. In detail, when an on-state
compensation control signal is input, a carrier feedthrough signal
is input to the first controller 900 and the second controller 910,
and when an off-state compensation control signal is input, a
carrier feedthrough signal is not input.
[0026] The fourth switching unit 800 periodically or sequentially
transmits a DC voltage with respect to a carrier feedthrough signal
to the first controller 900 and the second controller 910. A
selection signal (I/Q selection) about whether to control the
carrier feedthrough with respect to the signal of the in-phase path
or control the carrier feedthrough with respect to the signal of
the quadrature-phase is input in a certain period. The switching
units are achieved by switches in the exemplary embodiment of the
present invention, but are not limited thereto.
[0027] The first differential amplifier 200 and the second
differential amplifier 210 respectively supply a first bias voltage
(Vbias_1) and a second bias voltage (Vbias_2), and a first control
voltage (I_offset control) and a second control voltage (Q_offset
control), which are respectively output from the first controller
900 and the second controller 910, to the quadrature modulator 300.
Further, the first differential amplifier 200 and the second
differential amplifier 210 function as modulators, for
in-phase/quadrature-phase baseband signals output from the first
switching unit 100 and the second switching unit 110 by control of
an off-compensation control signal.
[0028] The quadrature modulator 300 includes a first mixer 310, a
second mixer 320, and an adder 330. The quadrature modulator 300
outputs, as quadrature modulation signals, an in-phase baseband
signal and a quadrature-phase baseband signal having a phase
difference of 90.degree. in response to in-phase/quadrature-phase
baseband signals output from the first differential amplifier 200
and the second differential amplifier 210.
[0029] The DC voltage difference of a differential input of each of
in-phase/quadrature-phase baseband signals input to the quadrature
modulator 300 is input to the first mixer 310 and the second mixer
320, and mixed with local oscillator signals (cos .omega. t and sin
.omega. t), respectively. Accordingly, limited carrier feedthrough
is caused in a radio frequency path. The functions of the first
mixer 310, the second mixer 320, and the adder 330 are already
known, and are not described in the exemplary embodiment of the
present invention.
[0030] The power amplifier 400 receives a feedthrough carrier
output from quadrature modulator 300, amplifies the power, and then
transmits the carrier to an antenna.
[0031] The coupler 500 is disposed between the power amplifier 400
and the antenna and extracts a portion of the carrier signal, which
is amplified by the amplifier 400 and transmitted to the antenna
through a transmission signal path, from the transmission signal
path. The carrier signal is composed of a quadrature-phase signal
and an in-phase signal.
[0032] The signal extracted by the coupler 500 is input to the
detector 600. In other words, the coupler 500 extracts a carrier
feedthrough signal from the transmission signal path such that the
detector 600 can detect the carrier feedthrough signal, a radio
frequency signal.
[0033] The detector 600 measures the amount of carrier feedthrough
by detecting the magnitude of the carrier extracted by the coupler
500. The detected carrier feedthrough signal is converted into a DC
voltage.
[0034] The first controller 900 and the second controller 910
periodically or sequentially receive the detected carrier
feedthrough signal from the fourth switching unit 800 and output a
control voltage for adjusting a voltage difference generated
between the in-phase signal and the quadrature-phase signal. The
first controller 900 and the second controller 910 compare the
received carrier feedthrough signal with the first reference
voltage (Vref_I) and the second reference voltage (Vref_Q),
respectively.
[0035] The first reference voltage (Vref_I) and the second
reference voltage (Vref_Q) respectively mean a differential DC
error voltage of an in-phase baseband input signal and a
differential DC error voltage of a quadrature-phase baseband input
signal for minimizing the carrier feedthrough. The first reference
voltage (Vref_I) and the second reference voltage (Vref_Q) can be
changed according to the system design because they depend on
characteristics of the component elements.
[0036] Next, the configuration of the first controller 900 and the
second controller 910 illustrated in FIG. 1 is described with
reference to FIG. 2.
[0037] FIG. 2 is a diagram illustrating the detailed configuration
of an apparatus for compensating carrier feedthrough according to
an exemplary embodiment of the present invention.
[0038] As shown in FIG. 2, the first controller 900 and the second
controller 910 may have two different types of configurations. For
example, there are a method of sequentially and alternately
performing in-phase/quadrature-phase control and a method of
time-divisionally performing in-phase/quadrature-phase control.
[0039] The method of time-divisionally performing
in-phase/quadrature-phase control is described in the exemplary
embodiment of the present invention. Describing control of
in-phase/quadrature-phase signals according to this method, the
first controller 900 includes a first filter 901, a fifth switching
unit 902, a first comparator 903, and a first integrator 904.
Further, the second controller 910 includes a second filter 911, a
sixth switching unit 912, a second comparator 913, and a second
integrator 914.
[0040] First, the first filter 901 receives a DC voltage with
respect to a carrier feedthrough signal output from the fourth
switching unit 800 and the second filter 911 averages a DC voltage
transmitted in a certain period in response to a carrier
feedthrough signal output from the fourth switching unit 800. The
first filter 901 and the second filter 911 may be RC filters, but
the first filter 901 and the second filter 911 are used only when
an RC time constant is larger than the time-division period. The DC
voltage with respect to the carrier feedthrough signal that is
input to the first filter 901 and the second filter 911 is input to
only one of the filters, depending on switching of the fourth
switching unit 800.
[0041] The fifth switching unit 902 and the sixth switching unit
912 receive the carrier feedthrough signal averaged by the first
filter 901 and the second filter 911 when an on-state compensate
control signal is applied, and the first reference voltage (Vref_I)
and the second reference voltage (Vref_Q) are applied to the first
comparator 903 and the second comparator 913, respectively.
[0042] The first comparator 903 and the second comparator 913 each
receive the carrier feedthrough signal detected by the detector 600
and then compare the carrier feedthrough signal with the first
reference voltage (Vref_I) and the second reference voltage
(Vref_Q). The first reference voltage (Vref_I) and the second
reference voltage (Vref_Q) mean a differential DC error voltage of
an in-phase baseband input signal and a differential DC error
voltage of a quadrature-phase baseband input signal for minimizing
the carrier feedthrough.
[0043] The first integrator 904 and the second integrator 914
respectively receive compared values output from the first
comparator 903 and the second comparator 913, that is, a difference
value of the DC voltage of the carrier feedthrough signal and the
first reference voltage (Vref_I), and a difference value of the DC
voltage of the carrier feedthrough signal and the second reference
voltage (Vref_Q), and then accumulate and output the difference
values. The accumulated values are respectively input to the first
differential amplifier 200 and the second differential amplifier
210 illustrated in FIG. 1. The differential DC voltage of the
baseband input that minimizes the amount of the carrier feedthrough
may be set to 0 or a certain level by the first reference voltage
(Vref_I) and the second reference voltage (Vref_Q).
[0044] Next, a circuit for achieving the first controller and the
second controller illustrated in FIG. 2 is described hereafter with
reference to FIG. 3.
[0045] FIG. 3 is a circuit diagram of an apparatus for compensating
carrier feedthrough according to an exemplary embodiment of the
present invention.
[0046] As shown in FIG. 3, the first filter 901 and the second
filter 911 are each achieved by resistors R1 and R2 and condensers
C3 and C4, respectively. Further, the first and second comparators
903 and 913, and the first and second integrators 904 and 914 are
each achieved by using one operation amplifier OP1 and OP2,
respectively.
[0047] First ends of the resistors R1 and R2 are connected to the
output terminal of the fourth switching unit 800 and second ends of
the resistors R1 and R2 are connected with the input terminals of
the operation amplifiers OP1 and OP2. Further, the second ends of
the resistors R1 and R2 are connected with the condensers C3 and
C4, and the other sides of the condensers C3 and C4 are
grounded.
[0048] In the exemplary embodiment of the present invention, the
comparator and the integrator are achieved by one operation
amplifier, but are not limited thereto. One input terminal of the
operation amplifier is connected with the output terminal of the
filter, and a reference voltage is input to the other terminal of
the operation filter.
[0049] Next, a method of compensating carrier feedthrough using the
apparatus for compensating carrier feedthrough having the above
configuration is described hereafter with reference to FIG. 4. The
operation for compensating the carrier feedthrough may be performed
once at an early stage of operation of a system, or every time
there is no signal input to the apparatus.
[0050] FIG. 4 is a flowchart illustrating a method of compensating
carrier feedthrough according to an exemplary embodiment of the
present invention.
[0051] As shown in FIG. 4, first, a plurality of switching units is
initialized to compensate carrier feedthrough (S100). In the
exemplary embodiment of the present invention, in order to
compensate the carrier feedthrough, by applying an on-state
compensation control signal, the first switching unit 100 and the
second switching unit 110 are grounded such that
in-phase/quadrature-phase baseband input signals are not input, and
the fifth switching unit 902 and the sixth switching unit 912 are
opened. Further, the third switching unit 700 is connected to the
fourth switching unit 600 such that the carrier feedthrough signal
is input to a feedback circuit, the fourth switching unit 800 is
set such that an in-phase signal and a quadrature-phase signal that
are carrier feedthrough signals are input in a predetermined
certain period into the first controller 900 and the second
controller 910.
[0052] Because the in-phase/quadrature-phase input switches of the
first switching unit 100 and the second switching unit 110 are
grounded, only the first bias voltage (Vbias_1) and the second bias
voltage (Vbias_2) are input to the first differential amplifier 200
and the second differential amplifier 210 at the early state and
compared with a predetermined initial level of voltage. Further, on
the basis of the bias voltage input at the initial state, after
control voltages are calculated by the first controller 900 and the
second controller 910, not only the bias voltage but also the first
control voltage (I_offset Control) and the second control voltage
(Q_offset Control) are respectively input to the first differential
amplifier 200 and the second differential amplifier 210, and are
transmitted to the quadrature modulator 300 through the
differential amplifiers 200 and 210.
[0053] The quadrature modulator 300 modulates the carrier
feedthrough signals created by the differences between the first
bias voltage (Vbias_1) and the second bias voltage (Vbias_2) and
the first control voltage (I_offset Control) and the second control
voltage (Q_offset Control) into radio frequency signals and then
outputs them to the power amplifier 400. The feedthrough carrier is
amplified by the power amplifier 400 and input to the coupler 500.
The process for the above operation is already known in the art and
so is not described in the exemplary embodiment of the present
invention.
[0054] The coupler 500 not only transmits the signal output from
the quadrature modulator 300 to the antenna, but also extracts a
portion of the power of the signal. A portion of the power of the
feedthrough carrier output from the quadrature modulator 300 is
extracted in this embodiment (S110). The carrier feedthrough signal
extracted from the coupler 500 is input to the detector 600 and the
detector 600 estimates the amount of the input carrier feedthrough
signal and then converts the estimated amount into a DC voltage
(S120). Thereafter, the detector 600 outputs the converted DC
voltage to the third switching unit 700 that transmits the
converted DC voltage to the feedback circuit.
[0055] As already set in the step S100, since the third switching
unit 700 is connected, the DC voltage input to the third switching
unit 700 is transmitted to the fourth switching unit 800, and the
fourth switching unit 800 feedbacks the DC voltage and then inputs
the fedback DC voltage into the first filter 902 and the second
filter 912 in a certain period (S130). The DC voltage transmitted
in a certain period is averaged by the first filter 901 and the
second filter 902 while the periodicity is removed (S140).
[0056] Thereafter, the first comparator 903 and the second
comparator 913 compare the first reference voltage (Vref_I) and the
second reference voltage (Vref_Q) where the amount of carrier
feedthrough is the minimum with fedback signals that are averaged
(S150). Further, the first integrator 904 and the second integrator
914 generate control voltages by integrating the compared results
and output the generated control voltages into the first
differential amplifier 200 and the second differential amplifier
210, respectively, such that converted control voltages are applied
(S160).
[0057] If the fedback signals do not agree with the reference
voltage, the first integrator 904 and the second integrator 914
repeat the steps S110 to S160. However, if the fedback signals
agree with the first reference voltage (Vref_I) and the second
reference voltage (Vref_Q), respectively, it is determined that the
amount of the carrier feedthrough is the minimum, such that the
fifth switching unit 902 and the sixth switching unit 912 are
connected by inputting an off-state compensation signal, and the
third switching unit 700 is opened such that the carrier is not
input (S180).
[0058] The first control voltage and the second control voltage
generated by the first integrator 904 and the second integrator 914
are applied to the first differential amplifier 200 and the second
differential amplifier 210. Thereafter, differences between the
control voltages and the bias voltages input to the first
differential amplifier 200 and the second differential amplifier
210 are adjusted such that the carrier feedthrough becomes the
minimum. As described above, the steps S110 to S160 are repeated
until the carrier feedthrough reaches the minimum level. The first
reference voltage (Vref_I) and the second reference voltage
(Vref_Q) are changed depending on characteristics of the quadrature
modulator in the system.
[0059] Next, in step S170, when it is determined that the carrier
feedthrough signal with respect to the in-phase/quadrature-phase
path converges to the level of the reference voltage and the amount
of carrier feedthrough becomes the minimum, the off-state
compensation control signal is input, such that the fifth switching
unit 902 and the sixth switching unit 912 are each connected.
Further, the third switching unit 700 is opened and applies the
same reference voltage to two input terminals of each of the first
comparator 903 and the second comparator 913 (S190).
[0060] In this case, the same signals may be predetermined signals.
Applying the same reference voltage to two input terminals of each
comparator is for generating, as outputs, pulses showing that the
comparators have the same positive/negative levels by inputting the
same signals to the first comparator 903 and the second comparator
913. This is for allowing the first integrator 904 and the second
integrator 914, which have received a pulse, to constant control
voltages that are not changed from the existing output control
voltages.
[0061] Therefore, pulses showing that the reference voltage and the
DC voltage have the same positive/negative level are generated as
the outputs of the first comparator 903 and the second comparator
913 (S200). This means that there is no difference between the
reference voltage and the DC voltage, that is, there is no carrier
feedthrough. In this case, the off-state compensation control
signal is input and the in-phase/quadrature-phase baseband signal
is input to the system.
[0062] In other words, when there is a carrier feedthrough signal,
the first comparator 903 and the second comparator 913 generate
pulses corresponding to positive or negative levels to show how
much difference is between the amount of carrier feedthrough signal
and the reference voltage, and the integrators generate control
voltages by integrating the outputs of the comparators. However,
when there is no carrier feedthrough, the first comparator 903 and
the second comparator 913 generate pulses that show they have the
same positive/negative levels.
[0063] The first integrator 904 and the second integrator 914
integrate the outputs of the first comparator 903 and the second
comparator 913 and output the integrated results, and accordingly,
predetermined control voltages are applied to the first
differential amplifier 200 and the second differential amplifier
210 (S210). This is for transmitting the predetermined control
voltages that are applied to the first differential amplifier 200
and the second differential amplifier 210 to the quadrature
modulator 300.
[0064] A quadrature modulation system having a quadrature-phase
signal and an in-phase signal was described in the exemplary
embodiment of the present invention, but the invention is not
limited thereto.
[0065] According to the present invention, it is possible to
suppress carrier feedthrough using a simple analog type apparatus
for compensating carrier feedthrough.
[0066] Further, it is possible to simply achieve an apparatus for
compensating carrier feedthrough using a variety of quadrature
modulators that require that a baseband differential input voltage
difference is 0 or has a slight voltage error.
[0067] The embodiment of the present invention described above is
not implemented by only the method and apparatus, but it may be
implemented by a program for executing the functions corresponding
to the configuration of the exemplary embodiment of the present
invention or a recording medium having the program recorded
thereon. These implementations can be realized by the ordinary
skilled person in the art from the description of the
above-described exemplary embodiment.
[0068] While this invention has been described in connection with
what is presently considered to be practical exemplary embodiments,
it is to be understood that the invention is not limited to the
disclosed embodiments, but, on the contrary, is intended to cover
various modifications and equivalent arrangements included within
the spirit and scope of the appended claims.
* * * * *